The present invention relates to a waveform generator particularly of the type which is suitable for the generation of sawtooth and/or triangular waveforms. The present invention also relates to a display device and to electronic apparatus.
Sawtooth and triangular waveform signals are used in many analog electronic circuits, including for example circuits providing pulse width modulation (PWM) and in analog-to-digital converters.
According to the present invention there is provided a waveform generator comprising an input terminal connected to one terminal of a first capacitor, a first switch connecting the other terminal of the first capacitor to one terminal of a second capacitor, a second switch connected for operatively discharging the second capacitor, the said one terminal of the second capacitor also being connected to she input of a buffer the output of which is connected to the output of the generator, and a third switch connected so as operatively to feed back the output of the buffer to the other terminal of the first capacitor.
Known circuits for generating sawtooth and/or triangular waveforms are based on single crystal MOSFET devices and have not been implemented using thin film transistors (Ms) due to problems incurred by TFT characteristics. It is an advantage of the present invention that the waveform generator can be implemented using TFTs.
The so-called kink effect experienced with TFT transistors and their lack of good saturation output characteristics are particular examples of the problems which have prevented the prior use of TFTs in waveform generator circuits.
Unlike the output characteristics (IDxe2x88x92VDS) of single crystal MOSFETs, with polycrystaline silicon TFTs a saturation regime is not observed. Instead when the device is operating above the so-called pinch-off level, generally when VDS greater than VGS, high electric fields are formed near the drain and this results in so-called impact ionisation. The result is an increase in drain current ID and it is this which is referred to as the kink effect. This effect increases power dissipation and degrades the switching characteristics in digital circuits, whilst reducing the maximum obtainable gain as well as the common mode rejection ratio in analogue circuits.